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1 Exercise Science, Department of Physical Therapy, Marquette University, Milwaukee, WI, USA
2 Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: Sandra.Hunter{at}marquette.edu.
The purpose was to compare the time to task failure for a sustained isometric contraction performed at a submaximal intensity with the elbow flexor muscles by young and old men and women. Twenty-seven young (14 men, 13 women, 18-35 years) and 18 old (10 men, 8 women, 65-80 years) adults sustained an isometric contraction at 20% of maximal voluntary contraction (MVC) torque until the target torque could no longer be achieved for at least 5 s. The young adults (66.8 ± 17.9 N.m) were stronger than the old adults (47.7 ± 18.1 N.m, P < 0.05) and the men (69.8 ± 17.9 N.m) were stronger than the women (47.1 ± 15.3 N.m, P < 0.05), with no interaction between age and sex (P > 0.05). The time to task failure was longer for the old adults (22.8 ± 9.1 min) compared with the young adults (14.4 ± 7.6 min, P < 0.05) and for the young women (18.3 ± 8.0 min) compared with the young men (10.8 ± 5.2, P < 0.05), but there was no difference between the old women (21.3 ± 10.7 min) and men (24.1 ± 8.0 min, P > 0.05) or between the young women and old adults (P < 0.05). Mean arterial pressure (MAP), heart rate, average electromyographic activity (EMG), and torque fluctuations of the elbow flexor muscles increased during the fatiguing contraction (P < 0.05) for all subjects. The rates of increase in MAP, heart rate, and torque fluctuations were greater for young men and old adults, with no differences between the old men and women (P < 0.05). Similarly, the rate of increase in EMG activity was greater for the young men compared with the other three groups. In contrast, the EMG bursts were less frequent for the older adults (P < 0.05) at the end of the fatiguing contraction and this was accompanied by reduced fluctuations in torque. Consequently, the time to task failure was related to the target torque for the young adults, but not for the old adults, and the differences in task duration were accompanied by parallel changes in the pressor response.
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